This invention concerns scribed interconnections and processes for producing same. More particularly, this invention concerns discrete wire interconnections and processes for scribing conductors on a surface of a substrate to form or to modify point-to-point electrical connections.
Processes that employ wires as the interconnection medium in the production of circuit boards are known. In one such process, known as the Stitch Weld.TM. process, a standardized circuit board panel is utilized. The panel is comprised of pads and circuit holes representing various component sizes and pin densities of components. Components to be used in the circuit are matched to fit existing pads and holes on the standardized panel. Interconnection is achieved by welding endpoints of each wire to pads on the panel. This allows wires to be looped across the board between endpoints. Component insertion in this process is difficult. Wires typically cover the entire board thereby blocking areas where components are to be placed. Automatic component insertion, in most cases, cannot be used because component leads would get tangled in the wires.
U.S. Pat. No. 4,414,741 to Holt attempts to alleviate the "looping" problem of the Stitch Weld.TM. process. Holt describes placing temporary pins on a circuit board panel to define channels which do not interfere with component locations. Wires are routed within these channels and wire endpoints are soldered to the appropriate pads. The temporary pins are removed and the circuit board is covered by a plastic sheet which encapsulates the wires to retain them in the channels. However, this process has a number of disadvantages. First, wires still loop across the surface of the board from ends of the channels to wire termination points. Second, because certain portions of the circuit board are dedicated to the channels, the potential component density of the board is greatly diminished. Third, special holes must be drilled in the circuit board substrate to accommodate the temporary pins. Lastly, electrical interference and signal distortion may arise because a multiplicity of wires are positioned closely together in the channels.
Floury, et. al., "An automatic wiring eguipment for hybrid substrates", Proceedings of the 32nd Electronic Components Conference, May, 1982, describe another process for construction of circuit boards, wherein a fine line of adhesive is first dispensed along the desired wire paths. Wires are subseguently routed over the adhesive to bond wires to the circuit board. Endpoints of the wires are then bonded to pads. This process is inefficient, as it reguires the additional step of routing all wire paths with adhesive before placement of the wires on a board. The step of placing adhesive on the board prior to wire placement doubles the manufacturing time of the process. In addition, when wires cross over one another, there is no adhesion between the wires.
U.S. Pat. Nos. 3,674,914, to Burr; 3,674,602, to Keogh and Canadian Pat. No. 1,102,924 granted June 9, 1981, all incorporated herein by reference, describe a process and apparatus wherein wire is scribed onto an insulating base. A strand of preformed wire is fed continuously onto a surface of the base. The wire is simultaneously affixed to the base surface and cut at a predetermined terminal point on the base thereby producing a predetermined interconnection pattern. In practice, and as described in the patent, the wires are adhered to the surface of the base by applying adhesive to the base surface and activating the adhesive when the wires are scribed to the surface of the base. A difficulty of this arrangement, however, is that when activated on the board, the soft adhesive may allow the wire to drift or swim before the wire becomes adhesively set. This may lead to short or open circuits when the wires are not in their desired positions on the board. These patents also suggest the possibility of applying adhesive to the wires prior to the scribing of the wire conductors. However, there is no teaching of how to practice this suggestion.
European patent application No. 113,820 published July 25, 1984, describes a process wherein wire is scribed on a photocurable layer on the surface of a circuit board. The photocurable coating which receives the wire is softened prior to or at the time of scribing and is subsequently photocured. In the preferred form, ultrasonic energy is used to soften the adhesive layer and light energy is used to cure the photocurable adhesive layer.
U.S. Pat. No. 4,450,623 to Burr describes a process wherein wires are scribed on an adhesive-coated surface of a substrate having etched conductive pads representing terminal and inflection points in the circuit. Where the wire contacts a pad, it is soldered or welded thereto. Heat and pressure is then applied to the board to activate the adhesive and to embed the wire conductors into the substrate surface. Burr mentions the possible use of wire coated with adhesive. As before, once the wire has been scribed and soldered, heat and pressure is applied.
Japanese Laid Open application No. 57-136,391, published Aug. 23, 1982, describes a process wherein wires are scribed in a predetermined pattern onto an adhesive-coated surface of a substrate. Laser energy is used to soften the adhesive prior to scribing the wires.
British Patent Specification No. 1,504,252, published Mar. 15, 1978 describes a method of bonding and soldering insulated wires in a predetermined pattern on an insulating base support to establish electric contacts between conductive zones on the base support. Adhesive used for bonding the wires is deposited on the surface of the base support as a film layer in which heated wires may be embedded. Alternatively, a dry film coated wire is suggested which can be made self adhesive by heating or passing through a suitable solvent.
Wired circuit patterns have been made using a heated roller to embed enamel coated wire into an adhesive-clad substrate. The endpoints of the wire are soldered to exposed solder-coated pads on the substrate's surface. The adhesive is carefully applied to the sustrate by silk screen prior to the embedding step so as not to contaminate the solder pads and holes.
In another process, pre-perforated adhesive sheets are placed on the surface of a substrate having surface pads and holes. The perforations expose predetermined areas in which holes and surface feature will be placed. Conductors are then placed on the adhesive surface. However, each different board reguires its own special perforation pattern, making it cumbersome to use.
Processes that employ wires as the interconnection medium in the modification of printed circuit, multilayer and wire-scribed circuit boards are also known. These modifications may be reguired for different reasons. A circuit interconnection sequence may have to be changed by the designer to accommodate newly designed functionality on the circuit board. A conductor may be omitted or a misconnection may be made during the design process. A circuit board modification also may be required to correct a defect on the board that occurred during manufacture.
Repairing or modifying circuit boards is more economical and efficient than redesigning or manufacturing replacement circuit boards. The boards are typically modified by cutting unneeded connections and using small insulated wires commonly called "jumper wires". This wire addition process requires manual soldering of the wire endpoints. In this process, wires are usually looped away from the board. As greater numbers of jumper wires are added to the board, the circuit board becomes an unwieldy mass of looped jumper wires, commonly called a "rat's nest". The "rat's nest" often impedes or prevents the use of automatic component insertion, automatic soldering equipment and automatic circuit board test equipment, thus requiring additional manual operations which are time-consuming, error prone, and costly.
This manual jumper wire approach is economical only in low volume, low circuit density applications, where only a few additions are necessary. When high volume circuit board producers are forced to use this process, it is extremely costly because it is labor-intensive. Additionally, this process does not result in repeatable precise geometrical wire placement on all boards in a manufacturing lot. Also, defined impedance is difficult to achieve with jumper wires and reproducible electrical characteristics may be difficult to maintain.
Developments in semiconductor technology have increased the functionality of semi-conductors, i.e., the number of operations performed by the semiconductor, while decreasing the size of the overall semiconductor package. This results in a greater number of contact points in a smaller area for interconnection. Because of the greater number of contact points, the circuit board must provide for a higher density of interconnect per unit area.
This increases the probability of committing errors during a manual modification process. The close proximity of contact points or circuit terminal points on high density boards increases the probability of unintentionally modifying an erroneous endpoint.
Webizky et. al., "Making 100,000 Circuits Fit Where At Most 6,000 Fit Before", [Electronics Vol. 52, No. 15, (8-2-79)] describe a process wherein one surface of a circuit board is dedicated to pattern modifications. Although this process may improve the circuit board repair time, an additional circuit layer must be added to the board to accommodate the changes. This increases the manufacturing time and cost as well as the size of the circuit board package.
Processes for automatically repairing/modifying have been proposed. These include automatic positioning devices, optical light spot indicators, and laser soldering eguipment. These processes have helped to increase productivity, but they are still heavily dependent on manual placement of conductors or jumper wires. All the automated processes require a smooth, flat surface for modification.
U.S. Pat. No. 4,327,124 to DesMarais describes a process for screen printing circuit modifications on an etched printed circuit board wherein a dry photoprintable film resist is laminated on the circuit board over the existing conductors, exposed, developed, and cured. To apply the modifying conductors, a metal loaded, conductive polymer ink is applied through a screen to the film resist layer in the pattern of the conductor modifications. The circuit pattern is then covered with a conductive metal powder while the conductive polymer ink is still wet. The powder is pressed into the ink and the composition is cured. After the curing step, solder is screen-printed over the copper ink. The solder and copper are then fused together in an infrared reflow soldering machine. This process has many disadvantages and has not been widely adopted.